1. Field of the Invention
The present invention relates to an ignition device for an internal combustion engine. More particularly, it relates to such ignition device to prevent erroneous operations of it due to noises in a signal circuit of the ignition device and to provide proper ignition characteristics.
2. Discussion of Background
FIG. 21 is a circuit diagram showing a conventional signal circuit having a noise mask function of an ignition device disclosed in Japanese Unexamined Patent Publication No. 6072/1985. In FIG. 21, a reference numeral 40 designates a generating coil, numeral 41 designates a diode for rectifying an output from the generating coil, numeral 42 designates a capacitor, numeral 43 designates a thyristor for ignition, numeral 44 designates an ignition coil, numeral 45 designates a spark plug, and numeral 46 designates a signalling coil connected to the gate of the thyristor 43 through a diode 47 and a resistor 48.
A numeral 49 designates a signal by-passing transistor whose base is connected to the generating coil 40 through a diode 50 which allows to pass a negative wave, while the base is supplied with a constant voltage from a power source circuit 51 through a resistor 52. A diode 53 and a resistor 54 are connected to the power source circuit so as to pass the negative wave generated from the generating coil 40.
The generating coil 40 generates an a.c. power by the revolution of a magnetic generator and each positive half wave charges the capacitor 42 through the diode 41. When the thyristor 43 receives a signal at its gate in an ignition period, the thyristor 43 is turned on so that an electric charge in the capacitor 42 is discharged to the primary winding of the ignition coil 44. Then, a voltage is induced in the secondary winding to result an electric discharge at the ignition plug 45 to thereby fire a cylinder of the internal combustion engine. The negative wave from the generating coil 40 flows through the diode 53 and the resistor 54, and a voltage corresponding to a voltage drop by the resistor 54 appears at both ends of the generating coil 40.
The power source circuit 51 is adapted to receive power from the generating coil 40 and outputs a constant voltage.
The signalling coil 46 generates a signal voltage depending on the revolution of the magnetic generator. However, when the generating coil 40 produces the positive voltage and is charging the capacitor 42, the output of the signalling coil 46 is by-passed and is not supplied to the thyristor 43 because a voltage from the power source circuit 51 is supplied to the base of the transistor 49 to render it in a conductive state. When the charging of the capacitor 42 is finished and the voltage in the generating coil 40 is reversed to generate a negative voltage, a voltage corresponding to a voltage drop of the resistor 54 is applied across the emitter-base of the transistor 49. When the emitter-base voltage is higher than the voltage from the power source circuit 51, the transistor 49 is turned off, whereby the voltage of the signalling coil 46 is applied to the gate of the thyristor 43 to thereby fire the engine. Namely, the conventional ignition device has such construction that the transistor 49 is in an ON state to by-pass a noise signal while the voltage of the generating coil is positive. Accordingly, the device is very effective when it is used for a system that ignition is carried out at every one cycle of a voltage waveform produced by the generating coil 40.
However, when the conventional device is used for a generator, for instance, having four poles in which one ignition signal is to be produced in every one revolution, the following disadvantage is found. Namely, since the generating coil generates an ignition signal in every two cycles, when the voltage of the generating coil becomes negative after charging of the capacitor in the first one cycle, the noise mask is not formed with the result of erroneous ignition.
In some types of the internal combustion engines, it is desirable to have a constant ignition timing from the starting of the engine to a high speed region. However, a voltage waveform produced by a signalling coil attached to the magnetic generator varies depending on an angular position and the peek value becomes high depending on increase in the revolution. Accordingly, some delay of the ignition timing in the starting region is inavoidable. To eliminate this disadvantage, there has been proposed a technique as shown in Japanese Examined Utility Model Publication No. 41618/1978 in which a bias voltage is applied to an output from the signalling coil so that a signal voltage is compensated in a low speed region. However, the technique is applicable only when the engine is started or the bias voltage is to be low, and accordingly, erroneous operations by noises in a signal line are unavoidable.
Further, in an internal combustion engine having more than two cylinders to which the conventional device is applied, when a noise is produced in the signalling coil in the time when the noise masking is removed, the noise causes an erroneous ignition. For instance, when the noise is produced in the second cylinder prior to the normal ignition of the first cylinder, the normal ignition does not take place.
Furthermore, in a multicylinder type ignition device, when a noise voltage is produced in the signalling coil in a cylinder other than the specific cylinder which is intended to cause ignition, during removal of the masking, a thyristor for the other cylinder is triggered prior to the normal ignition of the specific cylinder to thereby cause the erroneous operation.